1. Technical Field
The present disclosure relates to a solid-state image sensor, an image capture device including the solid-state image sensor and a method and program for controlling image capturing. More particularly, the present disclosure relates to a solid-state image sensor and image capture device which can capture an image at a different frame rate for a particular color component included in incoming light from the other color components thereof.
2. Description of the Related Art
A known solid-state image sensor includes a semiconductor layer in which photoelectric transducers such as photodiodes are arranged two-dimensionally and an array of color filters which is arranged closer to the light source than the semiconductor layer is. A color filter limits the wavelength of a light ray to be incident on each photoelectric transducer to a particular wavelength range such as the R (red), G (green) or B (blue) wavelength range. In each pixel of the solid-state image sensor, a light ray that has been transmitted through a color filter in the color allocated to that pixel (i.e., an R, G or B ray) is received at the photoelectric transducer. The photoelectric transducer generates a quantity of electric charge corresponding to the quantity of the light ray that has been incident on the pixel and then transmitted through the color filter of that pixel (which will be referred to herein as “incident light quantity”).
Those pixels are arranged in rows and columns on the image capturing plane of the solid-state image sensor. In order to address respective pixels and read a signal representing the quantity of electric charge that has been stored in a pixel of interest (i.e., a pixel signal), a lot of signal lines that are connected to those pixels and peripheral circuits that are connected to those signal lines are needed.
In a CMOS image sensor, a photoelectric transducer and a plurality of transistors to read a signal with a level representing the quantity of electric charge that has been generated and stored by the photoelectric transducer are arranged in an area corresponding to one pixel of the image capturing plane. Thus, one “pixel” of a CMOS image sensor is ordinarily made up of a single photoelectric transducer and multiple transistors.
In this description, a pixel in which a filter that transmits an R ray (i.e., an R filter) is arranged closer to the light source, a pixel in which a filter that transmits a G ray (i.e., a G filter) is arranged closer to the light source, and a pixel in which a filter that transmits a B ray (i.e., a B filter) is arranged closer to the light source will be referred to herein as an “R pixel”, a “G pixel” and a “B pixel”, respectively, for the sake of simplicity. Also, an image obtained from multiple R pixels that are arranged on an image sensor, an image obtained from multiple G pixels arranged there, and an image obtained from multiple B pixels arranged there will be referred to herein as an “R image”, a “G image” and a “B image”, respectively. These images can be obtained by reading pixel signals from a lot of pixels that are arranged on the image capturing plane. The image data is read on a frame-by-frame basis. And the number of times image data is read per second is called a “frame rate”.
In order to obtain a high-resolution, high-frame-rate moving picture using such a solid-state image sensor, not just the resolution needs to be increased by reducing the area of each pixel but also the frame rate needs to be increased by shortening the charge storage period (i.e., exposure time) of respective pixels. However, if the area of each pixel were reduced and if the exposure time were shortened, then the quantity of the light incident on each pixel would decrease. And since such a decrease in the quantity of light incident will lower the output level of a pixel signal, the SNR (signal to noise ratio) of a moving picture will eventually decrease, which is a problem.
In order to overcome such a problem, it was proposed that the R, G and B color components be captured at mutually different resolutions and in mutually different exposure times. For that purpose, a technique for separating incident light into R, G and B components and capturing images using two different image sensors for two groups formed by these color components is disclosed in PCT International Application Publication No. 2009/019823 and PCT International Application Publication No. 2009/019824. For example, if R and B color components are captured at a low resolution and a high frame rate, images with a temporally high resolution can be obtained for the R and B color components. Meanwhile, if the G color component is captured at a high resolution and a low frame rate, then the exposure time and spatial resolution required can be secured for the G color component and a sufficient quantity of light can be obtained. As a result, a G image with a high SNR can be obtained at a high spatial resolution. And if a high-resolution, high-frame-rate moving picture is restored by performing image processing on those color component images that have been captured at a low resolution and a high frame rate and the color component image that has been captured at a high resolution and a low frame rate, a high-resolution and high-frame-rate color moving picture can be obtained.
If images are captured using different exposure times and different frame rates on a color component basis as described above, pixel signals are output from the image sensor at mutually different times from one color component to another. That is why to perform such an image capturing process using a single-panel color image sensor, read signals are supplied to multiple groups of pixels associated with the respective color components at respective timings corresponding to their frame rates independently of each other and the pixel signals representing the respective color components are output independently of each other.
In order to set the frame rate for obtaining a G image to be lower than the frame rate for obtaining R and B images in a single-panel color image sensor, the time interval at which a signal representing the electric charge that has been stored in G pixels is read is set to be longer than the time interval at which a signal representing the electric charge that has been stored in R and B pixels is read. The article by Takeo Azuma et al, “A 2.2/3-inch 4K2K CMOS Image Sensor Based on Dual Resolution and Exposure Technique”, Proceedings in IEEE International Solid-State Circuit Conference 2010, pp. 408-410, 2010, discloses an image sensor which supplies pixel output read signals to the R, G and B pixels independently of each other and which can read signals from two pixels that are adjacent to each other in the column direction (i.e., vertically) in parallel with each other.
FIG. 25 illustrates a configuration for a single-panel image sensor as disclosed in the above article. In FIG. 25, the reference sign R denotes pixels that detect the intensity of the R component of incident light, the reference sign B denotes pixels that detect the intensity of the B component of the incident light, and the reference signs Gr and Gb denote pixels that detect the intensity of the G component of the incident light. In this image sensor, rows in which R and G pixels are alternately arranged horizontally (RG rows) and rows in which B and G pixels are alternately arranged horizontally (BG rows) vertically alternate with each other.
In this description, the G pixels of the RG rows will be referred to herein as “Gr pixels” and the G pixels of the BG rows will be referred to herein as “Gb pixels”. According to the above article, the exposure time of the G pixels is supposed to be longer than that of the R and B pixels and the G pixel signals are supposed to be output at a low frame rate. As shown in FIG. 25, read signal lines for passing read signals to respective pixels and output signal lines for passing the pixel output signals to the processing of the next stage, including A/D conversion, are separately provided for R, G and B pixels. As a result, as shown on the right-hand side of this drawing, two signal lines, namely one read signal line that is connected to the R or B pixels and another read signal line that is connected to the G pixels, run horizontally along each row of pixels. Meanwhile, two more signal lines, namely one output signal line that is connected to the R or B pixels and another output signal line that is connected to the G pixels, run vertically along each column of pixels. By adopting such a configuration, read signals can be supplied independently of each other to the R, B and G pixels and outputs can be obtained (i.e., signals can be read) in parallel with each other from the pixels in respective colors.